Noise reduction has emerged as a paramount criterion in the design of roller chain drive systems in recent years. It is generally recognized that in roller chain systems, the two most significant noise sources are from the polygonal action and intensive impacts due to relative velocity between the chain rollers and sprocket teeth during their meshing process. Polygonal action is the effect of the fluctuation of the position in which the chain and sprockets engage due to the wrapping of the chain in a polygonal manner, causing both longitudinal and transverse chain span vibrations. The vibrations result in unsteady chain speeds which affect the engagement process as well as the impact levels.
The effect of impact due to relative velocity (see FIG. 1) may be thus elucidated: the drive sprocket rotates at a constant velocity, and in converting rotational power to pulling power, the rollers are seated on the sprocket teeth in turn and follow the motion of the sprocket. At the moment that a sprocket tooth engages a roller, there is an instantaneous change in the direction of roller motion, resulting in an intensive impact.
In the prior art, standard approaches to improve the noise characteristics of chain-drive systems include increasing teeth number, manipulating parameters which affect the performance of the system and improving the roller-tooth engaging mechanism. Increasing teeth number is the most direct and effective method to reduce polygonal action, especially in the case of high speed chain drives. Design parameters which are commonly manipulated are the centre distance (which controls initial tension) and inertia of sprocket. The engaging mechanism can be improved by the use of intensified lubrication, silent chains, wheel guides and the like.
The aforementioned techniques improve the noise characteristics of chain-drive systems to an extent, but there are certain limitations. For instance, increasing the number of teeth requires an increased sprocket dimension, and installation of guide wheel requires additional space. Moreover, the methods by no means always function properly. In particular, the wheel guide installed in the tight span may guide the roller to move tangentially to the pitch circle of the sprocket under certain conditions, but not always. In general, the methods of the prior art are passive, i.e. they provide remedies based on the standard tooth profile design, which is a major drawback. An active approach would be to tackle the root of the problem of noise right from the initial stage of design, such as by designing a new tooth profile to minimise impact. As will be appreciated, the term “tooth profile” herein refers generally to the two-dimensional shape or profile of the sprocket tooth in a plane substantially perpendicular to the rotational axis of the sprocket.
The design of a new teeth profile is of no easy task, since design methods have been developed for many years and are recognized by various standards, such as ISO (International Standard Organisation) standard 606:1994(E) and JIS (Japanese Industrial Standard) B 1802-1981. These are reproduced in FIGS. 3–4, respectively. Nevertheless, there is still much scope for the development of tooth profile design and the art of tooth profile design to reduce noise has been substantially researched and refined in recent years.
U.S. Pat. No. 4,758,209, to Ledvina, describes a sprocket for use with a silent timing chain wherein the sprocket is formed with an even number of teeth of which there are alternating tooth profiles.
U.S. Pat. No. 5,397,278, to Suzuki et al, describes an improved sprocket which decreases the impact between the rollers and the sprocket during meshing. The vibration noise caused by polygonal action is not addressed.
U.S. Pat. No. 6,036,614, to Baddaria, describes a symmetrical sprocket tooth with a complex profile which addresses both impact noise as well as the noise arising from polygonal action. The profile has a roller scating angle exceeding that recommended by ISO, which may be a source of problems during disengagement. Moreover, the determination of the connecting point between the seating portion and impact portion is not straightforward and the complexity in defining the profile may also be objectionable, at least to designers.
A series of patents, U.S. Pat. No. 5,921,879, U.S. Pat. No. 5,993,344, U.S. Pat. No. 6,090,003 to Young, describe a roller chain sprocket having improved noise characteristics. The underlying principle is the provision of an asymmetrical tooth space that includes a flat surface, providing a “staged” roller impact wherein a tangential impact occurs first, followed by a radial impact. Although this technique splits a single impact into two occurring at different times, both the tangential and radial impacts are nonetheless instantaneous.
Therefore, an object of the present invention is to provide an improved sprocket with reduced noise generated when used in a unidirectional roller chain drive system.
A further object of the present invention is to provide an improved sprocket with reduced roller-tooth impact by designing a new tooth profile.